Posts Tagged ‘nuclear weapons’

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Analysis of Soviet smoke detector plutonium

February 7, 2017

Plutonium is a practical and versatile substance, having applications that range from planetary extinction to routine fire protection, depending on the user’s fancy.  The element has been mass-produced in nuclear reactors since World War Two, and occurs in various isotopic compositions in the discharged reactor fuel, reflecting variables such as fuel burnup, initial uranium enrichment, and neutron spectral characteristics of the reactor design.  The Soviet Union cooked up more plutonium than any other nation.  Most of this was slated for the noble purpose of containing capitalist imperialism, but some found its way into commercial ionization smoke detectors like the KI-1, RID-1, and RID-6M.  (The bourgeois warmongers themselves preferred, and still prefer, americium-241 for this application.) Occasionally, people in the former USSR try to peddle their old smoke detector plutonium on the nuclear black market, thinking that it may attract top dollar from terrorists with an appetite for nuclear warfare.  We’ll examine that possibility in more detail shortly.

Since I was curious about the technical characteristics of Soviet smoke detector plutonium, I picked up an old KI-1 smoke detector and sacked it for the source.  The source design bears much resemblance to the “lipstick” sense chamber sources in early Pyrotronics detectors made in the USA.  This one is brass and a bit wider than its Pyrotronics analogue.  An internal axial thread positions a cup-shaped alpha particle shield around a band containing the active deposit, thereby regulating the amount of ionization produced by the source in the chamber and controlling the sensitivity of the detector.  The sections below describe my analysis of the gamma and alpha radiations emitted by this source, and my conclusions about the plutonium’s age, activity, mode of production, and suitability for nuclear combat.

Plutonium isotopics by gamma spectrometry

High-resolution gamma spectroscopic measurements allow direct determination of the relative concentrations of Pu-238, Pu-239, Pu-240, and Pu-241 in a plutonium sample.  In such measurements, Pu-242 is customarily inferred from heuristic correlations to the other isotopes; it can be directly measured only with costly and destructive mass spectrometry.  Additionally, the ratio of daughter Am-241 to parent Pu-241 can be used to date plutonium.  The basic methodology is discussed in good detail in Sampson, T. E., Plutonium Isotopic Composition by Gamma-Ray Spectroscopy (1986).  I employed the multiple linear regression (MLR) formula of Sarkar, Shah, et al. (2014) to estimate Pu-242.

My gamma detector is a PGT n-type coaxial HPGe detector that lives in the guest bedroom of the home (the least radioactive room, as it should be), shielded with lead bricks and a graded inner shield of copper and tin sheet.  One preparation that is almost essential with plutonium is selective attenuation of the 59-keV gamma radiation from Am-241, as discussed in Sampson’s article above.  If you don’t do this, then the pileup and sum peaks caused by the intense Am-241 radiation will swamp the rest of the plutonium spectrum.  To hold the “lipstick” source, I made an attenuator out of rolled cadmium sheet and endcaps stuffed inside of a piece of copper water pipe with copper endcaps.  Such an arrangement works by strategically situating the K-edge energy of the absorber materials close to the energy of the offending radiation.  In quantitative gamma spec measurements, another important point is to avoid getting the source too close to the detector.  Otherwise, coincidences will distort the spectrum.  With the right attenuation and geometry, all that remains is to gather a statistically-useful number of counts in the spectrum–in this case, about 42 hours of counting.

The gamma spectrum is shown, annotated, in the gallery below.  It can be downloaded in ASCII format as an Excel spreadsheet here.  (Note that there are no channel numbers or energy calibration in the ASCII format, so you will have to add them.) As can be seen, Am-241 and Pu-239 peaks are scattered throughout, while Pu-238, Pu-240, and Pu-241 are represented by a single good peak each in the 150-keV neighborhood.  Am-241’s granddaughter Pa-233 is also in evidence, attesting to the unseen Np-237 daughter.  U-237 is a product of the minor alpha decay branch of Pu-241, and it interferes with some lines in the Am-241 decay spectrum as both nuclides decay to Np-237.  Those energies subject to interference cannot be used for quantitative analysis.   Click any image for the larger original:

Calculating relative activities from the peaks in the spectrum involves the following:

  • Measuring counts in each peak by peak-fitting algorithms.  I use the free software Hypermet-PC 5.12 to do this. Its algorithms are old, but well-known and still widely used.  Modern users will need to run it in DOSBox.
  • Correcting measured counts by an efficiency function of energy.  I fit this function in Hypermet-PC using a sealed Ra-226 source that can be placed in the same graded attenuator (and the same counting geometry) as the “lipstick” plutonium source.
  • Calculating relative activities from efficiency-corrected counts using the tabulated yields per decay of each radiation.  I used this website for my data.
  • Estimating Pu-242 activity using a suitable model.  My reference is here.

Once relative activities were established, I estimated total activities by comparing the gamma count rate on a Geiger counter between the KI-1 source and the ~60 microcurie Am-241 source from a Pyrotronics F-3/5A in the same counting geometry.  The overwhelming majority of the gamma rays emitted by both sources are 59-keV photons from Am-241.  These estimates are limited by the uncertainty surrounding the total activity of the Pyrotronics source.  The relative activities are known to much higher precision.  (I should note that the uncertainties given in the table relate to the relative measurements.)  As the table below illustrates, the KI-1 source contains a total activity of about 700 microcuries today, most of which is the 14-year weak beta emitter Pu-241.  The runner-up is 88-year alpha emitter Pu-238.  On an activity basis, the other nuclides are lower in the lineup.  The plutonium mass can be calculated, and it is about 1 mg.

The alpha spectrum

Alpha spectroscopy of plutonium is confounded by the fact that Pu-239 and Pu-240, and Pu-238 and Am-241, emit alpha particles with very similar energies.  The general technique is also laborious, involving chemical preparation of samples in virtually all cases.  Like Pyrotronics sources, there is some removable contamination on the KI-1 detector source.  I wiped a tissue on the source surface, ashed it, dissolved the residues in nitric acid, and evaporated them onto a stainless steel disc to make the spectrum shown below using an Ortec solid-state detector.  Despite this effort, it is not of great technical quality compared to what one could expect with a rigorous radiochemical technique.  All that said, though: the spectrum confirms the expectation of two main alpha energy groups, the larger at 5.4-5.5 MeV (Pu-238+Am-241) and the smaller at 5.1-5.2 MeV (Pu-239+Pu-240).

Dating plutonium using the Am-241:Pu-241 ratio

The Am-241:Pu-241 atom ratio is a daughter-parent ratio, a clock that allows us to date the plutonium.  More specifically, the method determines when Am was last chemically separated from the Pu, assuming that all the material in the source traveled together through the same process.  (The assumption may not be very good if multiple batches of Pu were mixed.)  A graphical solution of the coupled Bateman equations modeling Am and Pu ingrowth and decay is shown below.  The sample age is the point on the horizontal axis where the solution intersects the measured value of Am-241:Pu-241, represented by the one-standard-deviation band between the red and blue lines.  This plutonium appears to be 44.9 ± 0.4 years old, meaning it was probably processed in 1972.

Other dating ratios

Another member of the Pu-241 decay chain, Pa-233, can also be used for dating.  In its ratio with Am-241, we get an estimate of 55.4 years; in its ratio with Pu-241, we get an estimate of 48.2 years.  The Am-241:Pu-241 method above predicted 44.9 years.  These three ages would be harmonized if there were a bit more Am-241 in the mix, specifically about 18% more, suggesting that some may have been removed in the earlier history of the sample.  The removal may have coincided with initial fuel processing delayed appreciably after fuel discharge from the reactor, or it may have been undertaken some time after the initial processing.  I am in favor of a view that americium was last chemically separated about four years after fuel discharge, the fuel itself being about 49 years out of the reactor (discharged in 1968), and that the separatory chemistry in the early 1970s was selective for Am and largely left ingrown Np-237 (parent of Pa-233) with the Pu.  This hypothesis harmonizes all three age estimates.

Original plutonium composition

Armed with an age estimate and current activity ratios among all the Pu isotopes, the calculation of mass composition at the time of preparation is straightforward using tabulated values of the half lives (or decay constants) of the isotopes.  Once again, there are assumptions in this calculation and in the conclusions derived from it.  The most important is probably that the plutonium was “fresh” when it was processed (or, more specifically, that the time difference between when irradiation stopped and when processing occurred was small enough to be insignificant to the isotopics).  Is that a good assumption?  Because the half-life of Pu-241 is only 14 years, and because the logistics of nuclear fuel processing usually dictate several years of cool-down during which time the fuel is in storage, transit from the reactor, and standing in queue for processing, this number is perhaps most suspect–and we would expect its calculated value and that of the correlated Pu-242 estimate to err on the low side.  Keeping this caveat in mind, here is the composition of the original KI-1 smoke detector plutonium as calculated from the Am-241:Pu-241 age:

What if the plutonium is actually four years older (1968) and was just processed in 1972, as the Pa-233 dating methods hint?  Then, the composition looks like the table below.  I believe this is more accurate:

Conclusions: Low-burnup, reactor-grade plutonium from 1970 is nothing to fear

With original Pu-240 concentration near 20%, the ~1 mg of plutonium used in this Soviet KI-1 smoke detector falls into the “reactor grade” classification rather than “weapon grade.”  The classification convention distinguishes plutonium compositions on the basis of Pu-240 content because of this isotope’s high spontaneous-fission neutron yield and its consequences for pre-initiation in nuclear weapons.  However, weapons made from reactor-grade plutonium are known to work.  Their yield may not be statistically reliable or as high as could be expected with weapon-grade fissile material, but they are useful weapons nonetheless.  The real barrier to would-be proliferants hoarding Soviet smoke detectors is the sheer number–millions!–of the motherfuckers they would in principle need to acquire through the typical nuclear smurfing networks.  (The entire output of the Soviet smoke detector industry is unlikely to have involved more than one formula quantity of plutonium.)

Now that we can sleep easily on the nuclear holocaust issue, I’ll add a few more observations about this plutonium.  Although reactor grade, its high fraction of Pu-239 and low fractions of Pu-241 and Pu-238 suggest moderately low burnup, probably not in excess of 5 GWd/t, in a reactor amenable to such light utilization (e.g. an isotope production reactor or online-refuelable type).  The measured dates of production (1968) and last separation (1972) rule out VVER and RBMK power reactors as sources.  Some of the RBMK’s graphite-moderated, low-enrichment-fueled predecessors designed for isotope production and co-located with processing plants (such as the ADE types) are likely origins.  These reactors also turned out a weapon-grade stream as the USSR frantically raced for nuclear parity with the Yankee imperialists.

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Gamma Analysis of Chagan “Atomsite”

August 19, 2012

Lake Chagan (“Atomic Lake”) was formed in 1965 following a thermonuclear cratering explosion on the Semipalatinsk Test Site in Kazakhstan.  More photos from my recent trip to the site are here.  Merely visiting the site does not answer some of the most interesting questions about its current state, such as the isotopic origin of the significant (1-2 mR/hr) gamma radiation.

I decided to take a more scientific look at the gamma rays emitted from Chagan’s fused rock—the glassy, vesiculated slag (“atomsite” or “kharitonchiki”) that covers the ground near the shore of the lake.  A grab sample was acquired, and transported home by means other than my own return flight from Almaty (this airport’s departure lounge is guarded by a notoriously-sensitive portal scintillator made by Aspect).  I filled a 3-ounce plastic jar with the material for counting.

My method of analyzing this unique “soil sample” is HPGe gamma-ray spectrometry.  I followed the same approach discussed in my earlier analysis of Japanese soils, involving comparison of the test specimen with an identically-shaped Cs-137 sand standard.  My germanium detector is operated via a homebrew LabVIEW program built around Mark Rivers’ EPICS interface for the Canberra 556 AIM MCA and Carsten Winkler’s CA Lab; I subsequently analyze the spectra (peak fitting, background subtraction, energy calibration) with FitzPeaks.  In this experiment I collected an 8000-second count of the slag sample and a 2000-second count of the Cs-137 sand standard.  An appropriate long-duration background was subtracted from each.  The quantitative calculation of activities relies on a single major line from each nuclide, chosen (to the extent possible) to be close to 662 keV.  Corrections for detector energy response were made by calibrating the energy-dependent photopeak efficiency in FitzPeaks to a point Ra-226 source, covering the range of roughly 200-1600 keV with a power-law model.  Corrections for material attenuation, including density variations from the standard, are NOT made from a calibration but are calculated based on an exponential attenuation model that assumes the sample has the elemental composition of concrete.  It’s probably not a bad comparison, and typically results in a correction of under 20%.  However, I expect better accuracy in the quantitative analysis for peaks that are closer to 662 keV.  Finally, no corrections are made for count losses to coincidence summing.  An Excel spreadsheet of this data and analysis may be downloaded here.

Referring to the 0-1600 keV gamma spectrum below, the first major observation is that most of the lines belong to europium isotopes, Eu-154 and Eu-152.  These isotopes were produced when neutrons from the “device” were captured by the ~1ppm naturally-abundant Eu-153 and Eu-151, respectively, which have remarkably high capture cross-sections.  These activation products are also long-lived enough to persist in significant quantity to the present day.  The other major long-lived gamma-emitting activation nuclide is Co-60.  Some of this cobalt could be from metal in the bomb’s well casing, but it could also be from activation of crustal mineralization.  The remaining major activity, Cs-137, is a product of fission in the bomb’s fissionable components.

Gamma spectrum of Lake Chagan atomsite

If we examine the smaller peaks in detail (click on below thumbnails), long-lived isotopes of holmium (Ho-166m), silver (Ag-108m), and barium (Ba-133) are in evidence.  Am-241 is present at a low concentration; on the basis of its 59-keV gamma line I cannot confidently estimate its concentration using the Cs-137 reference source technique.  Am-241 is the daughter of Pu-241 produced by neutron capture on plutonium in the bomb, and thus is a reliable proxy for the presence of plutonium in the sample.  The gamma radiations from plutonium itself are too weak and swamped by the spectrum’s low-energy continuum to be observed.

The chart below presents the results of the quantitative analysis.  Gamma-emitting radionuclide activity in “Chaganite” exceeds 375 Bq / g, with Eu-152 being the most concentrated.

Nuclide concentrations, July 30 2012

Chaganite versus Trinitite: when the activities are normalized to their initial values at the time of the respective explosions (1965 and 1945), a direct comparison can be made that illustrates just how much more radioactive the Chagan slag is (see beow).  The data for Trinitite is taken from Pittauerova, Kolb, et al., “Radioactivity in Trinitite: a review and new measurements,” Proc. 3rd Eur. IRPA Conference, Helsinki, 14-16 June 2010.

Comparison of “Chaganite” with Trinitite

The Chagan slag contained almost an order of magnitude more Cs-137 at the time of formation, but it is the rather staggering ratios of the activation nuclides that surprises me the most: 400 times as much Eu-154 in Chaganite versus Trinitite.  70 times as much Eu-152.  And 370 times as much Co-60.  Why?  One fairly obvious explanation is found in the facts that Chagan was a more powerful bomb, detonated in closer proximity to the crustal rock that its neutrons activated since it was underground.  Some further considerations may also be relevant.  According to Carey Sublette’s Nuclear Weapons Archive, Chagan “was reported to be a low-fission design, which had a pure thermonuclear secondary driven by a fission primary with a yield of about 5-7 kt.”  In contrast, the Trinity bomb was a pure fission core surrounded by a uranium tamper.  Thus, escaping neutrons with a hard DT fusion spectrum probably carried a significantly higher fraction of Chagan’s energy yield relative to Trinity’s.

There is not a statistically-different concentration of Ba-133 between the two slags.  I think most of Trinity’s Ba-133 came from the bomb’s explosives, while Chagan’s probably came from crustal concentrations of barium.

Finally, if the Trinity bomb had a fission yield more than three times larger than Chagan, why is the latter’s concentration of Cs-137 higher?  The best reason I can suggest is Chagan’s better underground containment of volatile fission products.  In a surface explosion, volatile Cs and its beta-decaying precursors exist as gases for a long time, enabling atmospheric dispersal.  In an underground explosion, volatiles are condensed rapidly near where they were formed.

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Visit to the Semipalatinsk Nuclear Test Site

August 13, 2012

Soviet Ground Zero

At 7:00 on the morning of August 29, 1949, a nuclear fireball lit up the skies over a desolate expanse of steppe about 100 km from Semipalatinsk in the Kazakh Soviet Socialist Republic.  This explosion—the culmination of a research effort personally supervised by fearsome NKVD chief Lavrenty Beria—earned the Soviet Union status as a nuclear-armed superpower to rival the United States.  Over the course of the next 50 years, 615 more nuclear explosions, as well as numerous subcritical, radiological, and reactor-based experiments, occurred on the same New-Jersey-sized reservation—the Semipalatinsk Test Site.  The STS was largely abandoned in 1991 in the turbulent prelude to Kazakhstan’s independence.

This July I had the good fortune to visit the STS and its formerly-secret support city, Kurchatov.  Physical access to the STS is minimally controlled, but given the Kazakhstani police behaviors we observed, foreigners would arouse decidedly too much suspicion traveling to the area without official sanction for their trip.  Some reactors remain operational and some testing grounds (particularly Degelen) contain proliferation-sensitive debris.  I recommend contracting with a registered adventure tour company (I hired Nomadic Travel) to handle permissions, lodging, and appropriate transportation.  Roads on the Test Site are impassible in wet weather, merely brutal when dry, and I don’t exaggerate in the judgment that some of them would be faster on horseback!

Photo selections below include Kurchatov; Soviet “Ground Zero;” the aerial bombing target for the first Soviet staged thermonuclear bomb; the Degelen Mountain underground test site; a borehole on the Balapan underground site which experienced an “emergency situation;” and finally, the radioactive crater known as Lake Chagan.  The photos provided below are all captioned with additional detail.

Links to photo galleries (or scroll down):

  1. Kurchatov, the Secret City
  2. Soviet “Ground Zero”
  3. The RDS-37 Site
  4. The Degelen Mountains
  5. Borehole 1007: “Emergency Situation”
  6. Lake Chagan, the “Atomic Lake”
  7. Reactor Facilities at STS

Kurchatov, the Secret City

Kurchatov appeared on no maps and had no name (except for a cryptic post office number) for most of its existence. It was built hastily by GULAG labor and hosted many famous (and infamous) people of importance to the Soviet nuclear weapons project. Now it has a new life as a peaceful nuclear city, with a satellite campus of Kazakhstan’s National Nuclear Center occupying new buildings in town. Meanwhile, historic structures are crumbling and the town is clearly a shadow of its former self.

Soviet Ground Zero

60 kilometers southwest of Kurchatov is the 20-km-diameter “Experimental Field” (Опытное Поле), dotted with strange and dilapidated structures, radioactive slag, and swampy craters.  Its P-1 site, shown in all the photos below, was Ground Zero for “Joe-1″, RDS-6S (the first Soviet thermonuclear bomb, named for a delightful Russian pastry), and two other successful bombs.  All bombs tested at this spot were positioned on 15-30m towers.  At least two dozen more surface tests took place elsewhere on the Experimental Field.  To watch a video of “Joe-1,” click here.  To watch a video of the “sloika,” click here.

The RDS-37 Site

On Nov. 22 1955, the Soviet Union’s first multi-stage hydrogen bomb (embodying what is known as the “Teller-Ulam” configuration in the US, credited as Andrei Sakharov’s “Third Idea” in the Soviet Union) was dropped from an airplane toward a target designated by an 800-meter-diameter chalk circle on the Experimental Field about 3 km southwest of the P-1 site.  The bomb detonated at an altitude of 1.6 km with an unexpectedly-high yield of 1.5 megatons, killing a number of people in the region (including a 3-year-old girl). What remains today are faint traces of the target markings. Like the Nazca Lines, these are easier seen from space (see the Google satellite pic). Radiation levels at the site are modest, no more than about twice regional background.  There is no notable “atomsite” slag on the surface of this site.  Watch video of the RDS-37 blast here, which shows some footage of the event as seen from Kurchatov at the end.

The Degelen Mountains

“There’s plutonium in them thar hills!”  The Degelen Mountains were used for hundreds of underground nuclear tests carried out in horizontal adits in the rock. These adits are now “prohibited areas” because many tests were subcritical and chunks of plutonium remain in the residues that the Soviet Union neglected to clean up.  According to William Tobey’s sources, “hundreds of pounds of weapons-grade fissile material was ‘readily recoverable’ in the tunnels” at Degelen, enough to make quite a number of bombs.  The mountains themselves are hauntingly beautiful, and the surrounding foothills dotted with military ruins.

Borehole 1007: “Emergency Situation”

Borehole 1007 at the Balapan site was supposed to contain a routine underground nuclear test in February of 1972. But the bomb was a little too feisty, and ended up blowing the top off the well. A piece of the well casing (quite radioactive, I should mention) is now displayed in the STS Museum in Kurchatov. The rest of the well, and all its radioactive ejecta, is right here where we found it on the steppe.

Lake Chagan, the “Atomic Lake”

An idyllic and suspiciously-round lake of some 10 million cubic meters capacity graces the left bank of the Chagan River. It owes its existence to a 140-kiloton “peaceful” nuclear explosion carried out on January 15, 1965. The stated objective was to experiment with changing the course of rivers. Chagan was a filthy test, heavily contaminating the surroundings with radioactive byproducts. Like the American Operation Plowshare, bomb developers found that these peaceful uses worked after a fashion, but resulted in contamination that tended to preclude practical use. Lake Chagan would make a great picnic spot, but we were not able to enjoy some nourishment here ourselves because we were required to wear respirators over our pie-holes. The banks of Lake Chagan are strewn with this bomb’s unique slag, a sort of foamy, pumice-like rock.  Hottest spots on the bank now seem to be about 2 mrem / hour.  Click here to watch a video of Lake Chagan’s creation, including footage of swimmers in the water.

Reactor facilities

The Semipalatinsk Test Site contains more than just old nuclear weapons tests; it is also home to some working nuclear facilities that are quite fascinating. We didn’t make it inside the Baikal and IGR complexes, but I grabbed some photos in their general direction.

For more photos, including photos from the Tien Shan Mountains, Astana, Almaty, and other cities in Kazakhstan, please see my Facebook page.

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More radioactive goodies from Bayo Canyon

March 2, 2011

I’ve written about this place twice before, and a bumper crop of radioactive souvenirs from a February visit compels my new assessment that Bayo Canyon, New Mexico is simply unmissable for any hardcore nuclear tourist.  Of course, there’s the historical dimension:  the radiolanthanum experiments that commenced here in 1944 provided crucial insight into the implosion weapon design validated in 1945 by the Trinity test (and embodied later by “Fat Man” and virtually all successive bombs).  But what makes Bayo so special is that the history here is tangible, collectable, and detectable provided you come with a Geiger counter.

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The next four photos at left show pieces of blast debris that were scattered across the surface near the escarpment under Point Weather (where I am standing, 2nd photo above), along with readings in counts per minute on a Ludlum 44-9 pancake GM tube.  While the great majority of findings are not detectably hot, there is so much debris available that the prospects for major finds here are good.  This is my second piece of radioactive cable, and the other two pieces appear to be aluminum metal.  For comparison, local background is about 60 CPM.

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There is sufficient gamma radiation to identify uranium in one of these samples by scintillation spectrometry and to estimate its present activity.  The piece of cable was my choice for this test, owing to easy source-detector geometry and negligible self-absorption.  The last image is the 2000-second NaI:Tl gamma energy spectrum.  The peaks are consistent with the prominent decay radiation of U-235 at 185.72 keV (emitted in 57.2% of decays).  Assuming a geometric efficiency of ~50% and an intrinsic photopeak efficiency of ~75%, the piece of cable contains about 8 mg of uranium if the uranium has its natural isotopic ratio, or about 20 mg if it is depleted. (Both DU and natural U were used in the Bayo experiments.)

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